The present invention relates to metal hydride electrodes for alkaline electrochemical cells. In particular, the present invention relates to a metal hydride electrode having a metal oxide or a metal sulfide surface layer.
Rechargeable electrochemical cells using a hydrogen absorbing alloy as the active material for the negative electrode are known in the art. The negative electrode is capable of the reversible electrochemical storage of hydrogen. The positive electrode typically comprises a nickel hydroxide active material. The negative and positive electrodes are spaced apart in an alkaline electrolyte, and a suitable separator (i.e., a membrane) may be positioned between the electrodes.
As shown by reaction (1), upon application of an electrical current to the negative electrode, the hydrogen absorbing alloy (M) is charged by the absorption of hydrogen to form a hydride (Mxe2x80x94H).
M+H2O+exe2x88x92xe2x86x92Mxe2x80x94H+OHxe2x88x92(Charging)xe2x80x83xe2x80x83(1)
During discharge, the stored hydrogen is released by the hydride to provide an electric current and participates in electrochemical reaction, as shown by reaction (2).
xe2x80x83Mxe2x80x94H+OHxe2x88x92xe2x86x92M+H2O+exe2x88x92(Discharging)xe2x80x83xe2x80x83(2)
Examples of hydrogen absorbing alloys are disclosed in U.S. Pat. Nos. 4,551,400, 4,728,586, 5,096,667, 5,135,589, 5,277,999, 5,238,756, 5,407,761, and 5,536,591, the contents of which are incorporated herein by reference.
The reactions at a conventional nickel hydroxide positive electrode as utilized in a nickel-metal hydride electrochemical cell are as follows:
Ni(OH)2+OHxe2x88x92xe2x86x92NiOOH+H2O+exe2x88x92(Charging)xe2x80x83xe2x80x83(3)
NiOOH+H2O+e xe2x86x92Ni(OH)2+OHxe2x88x92(Discharging)xe2x80x83xe2x80x83(4)
Hence, as shown by reactions (1) and (2) above, the charging and discharging of the electrochemical cell involves the hydriding and dehydriding of the hydrogen storage alloys. Generally, the hydriding and dehydriding reactions in the electrochemical cell are accompanied by electrochemical charge transfer. These reactions also involve the transport of hydrogen atoms into and out of the hydrogen absorbing alloy. During the operation of the cells, particularly during high rate charge and discharge, significant hydrogen pressures can develop.
Specifically, during cell charging, atomic hydrogen is formed at the surface of the negative electrode. Preferably, the atomic hydrogen reacts with the hydrogen absorbing alloy as shown by reaction (1) to form a hydride. However, depending on charge conditions and surface properties of the hydrogen absorbing alloy, some of the atomic hydrogen may instead recombine with another atomic hydrogen to form molecular hydrogen gas and increase the internal pressure of the electrochemical cell.
Furthermore, in a typical nickel-metal hydride electrochemical cell, the negative electrode has an effective specific capacity which is greater than that of the positive electrode. Hence, during charging, the positive electrode reaches full charge before the negative and begins to evolve oxygen. The oxygen diffuses through the separator to the negative electrode. At the negative electrode some of the oxygen reacts with and discharges the metal hydride material to produce water. However, a portion of the oxygen fails to recombine in this manner but instead oxidizes the negative electrode which reduces the cycle life of the electrochemical cell. The remaining oxygen stays in the interior of the cell where it contributes to the overall increase in internal cell pressure.
While cells typically operate at pressures greater than atmospheric pressure, excessive hydrogen pressure and/or oxygen gas pressure is undesirable since it can result in a loss of aqueous-based electrolyte material, thereby limiting cell life. Also, if excess gas pressure is not vented, the cell can burst, deform, or otherwise be destroyed.
Clearly, it is desirable to limit excessive gas overpressure in electrochemical cells. Reduction of the internal cell pressure increases the cycle life of the electrochemical cell. The present invention relates to a novel layered electrode for the negative electrode of an alkaline electrochemical cell which reduces cell pressure within the electrochemical cell.
An objective of the present invention is to reduce the cell pressure within an alkaline electrochemical cell. Another objective of the present invention is to increase the cycle life of an alkaline electrochemical cell. Still another objective of the present invention is to increase the output power of an alkaline electrochemical cell.
These and other objectives are satisfied by a layered electrode for an alkaline electrochemical cell, comprising: a base hydrogen absorbing alloy electrode including a hydrogen absorbing alloy active material; and an outer layer affixed to at least a portion of the surface of said base electrode, said outer layer comprising at least one material selected from the group consisting of a metal oxide, a metal sulfide, and mixtures thereof.
These and other objectives are also satisfied by an alkaline electrochemical cell, comprising: a least one negative electrode; a least one positive electrode; and an alkaline electrolyte, wherein said negative electrode is a layered electrode comprising: a base hydrogen absorbing alloy electrode including a hydrogen absorbing alloy active material, and an outer layer affixed to at least a portion of the surface of said base electrode where said outer layer comprises a material selected from the group selected from a metal oxide, a metal sulfide, and mixtures thereof.